This invention relates to servo control systems for use in electro-mechanical printing devices of the type employing a rotary print wheel, sometimes termed a "daisy wheel printer", wherein the print wheel is both rotated and translated by a pair of motors under control of an electronic control system.
Rotary printing systems having servo control are known in which the rotary print wheel is mounted on a carriage for translation across the width of the print throat area during printing. The carriage is typically translated in right and left directions through a series of cables and pulleys driven incrementally by a DC motor controlled by a suitable electronic control system. As the carriage-wheel assembly is translated from character position to character position along the print line, the print wheel is rotated so that the character pad bearing the next character to be printed is aligned with the striking end of a print hammer, also mounted on the carriage, when the carriage is momentarily decelerated to a rest position. When the carriage has reached the rest condition, the print hammer is actuated to impress the character borne by the pad against an inking ribbon and the face of the printing media to print that character. After the print hammer rebounds, the carriage is translated to the next character location, the print wheel is rotated so that the proper character pad is aligned with the print hammer, and the next character is printed. This process continues until a complete line has been so printed, after which the carriage motion is reversed to print the next line of characters in reverse order, or the carriage is returned to the left-most starting position in preparation for printing the next line of characters.
Proper operation of such rotary printing systems requires an accurate servo control system for both the print wheel and the carriage. The servo system most typically used is a dual mode system having a velocity mode and a position mode. In the velocity mode, the velocity of the controlled element (i.e., print wheel or carriage) is controlled in accordance with a pre-selected velocity profile to insure optimum initial acceleration to a maximum design velocity value, followed by stepped deceleration until the desired linear or rotary position is achieved. Once this position (frequently termed the HOME position) has been reached, the servo is switched to a position mode of operation in which the linear or angular position of the controlled element is maintained substantially constant. In both modes of operation, position feedback signals generated by a position encoder associated to the controlled element (typically an optical encoder for generating sinusoidal position signals) are used to provide the necessary feedback information specifying the instantaneous position of the controlled element. These signals, either in their pure sinusoidal form or in logically processed pulse form, are coupled to a control unit in which the position information is used to determine certain key parameters, such as direction of print wheel rotation, incremental linear or angular distance from the present position to the next desired position, actual velocity of the controlled element, required incremental velocity, and the like. In addition, the control unit supervises the operating mode of each servo system, i.e., whether velocity or position mode, and generates the necessary servo control signals for switching the operation between the two distinct modes. In addition, when in the position mode the servo system uses one of the analog position feedback signals to monitor excursions of the controlled element away from the desired HOME position in order to generate corrective position signals to the motor driving the controlled element in order to counteract any such deviations.
One major problem or disadvantage with known dual mode servo systems used in rotary printing systems lies in the gain value selected for the system as a whole. Specifically, in order to provide the relatively fast response to changes in position and velocity of the controlled element, the known servo loops typically employ a fixed relatively high gain. While operation of the servo system with relatively high gain provides the fast response time required for most rotary printing applications, it suffers from the disadvantage that even slight excursions of the controlled element from the home position when the servo is in the position mode will cause oscillations of the controlled element about the home position during long detent pauses of the controlled element, such as when the printer is awaiting new character information after completion of the previous text.